It came after Dr Merryl Vorster told the court Pistorius has generalised anxiety disorder (GAD), and is a "distrusting and guarded" person who is "hyper-vigilant" about security.

But Mr Nel questioned the timing of the defence evidence.

Dr Vorster had been consulted and called only after Pistorius had given evidence to the court.

Mr Nel said: "Can it not be seen as a fallback position.

"The timing of the witness being called and the timing of the consultations is important."

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If Pistorius were found to be suffering from a mental illness, he could be held not criminally responsible for his girlfriend Reeva Steenkamp's death and found not guilty by reason of "mental illness or intellectual disability".

The law allows a person to undergo mental health observation for up to 30 days.

Mr Nel argued it was in the interests of justice that if there was any suggestion psychiatric factors had played a part in the killing, then the accused should be sent for observation.

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He has applied to the court for a referral.

But Pistorius' defence team said the application had "no merit" and was premature.

The judge has adjourned the trial to consider the prosecution's application, and will announce her decision on Wednesday morning.